Previously building with JDK > 1.8 b88 caused test failures due to errors
with custom compilers like Jibx and Jasper reports.

This commit adds a new TestGroup named CUSTOM_COMPILATION that allows the
CI server to continue to run these tests but allow committers to ignore
these tests.

Add a test to ensure that the CI server does not accidentally build
with the wrong JDK version.

Issue: SPR-10569

This commit introduces TestGroup#JMXMP and adds assumptions to related
tests accordingly. These tests require the jmxoptional_remote jar on the
classpath, and are run nightly in the SPR-PERF build.

Issue: SPR-8089

 - Add TestGroup#LONG_RUNNING to distinguish from #PERFORMANCE, the
   former being tests that simply take a long time vs the latter being
   tests that are actually dependent on certain actions happening within
   a given time window and are thefore CPU-dependent.

Issue: SPR-9984

Move code from spring-build-junit into spring-core/src/test along with
several other test utility classes. This commit removes the temporary
spring-build-junit project introduced in commit
b083bbde.

Introduce new 'spring-build-junit' subproject containing shared
JUnit utilities and classes to be used by other test cases. This
project is for internal use within the framework, and therefore
creates no artifacts to be published to any repository.

The initial code includes support for JUnit Assumptions that can
be used to determine when a test should run. Tests can be skipped
based on the running JDK version, logging level or based on specific
'groups' that have activated via a Gradle property.

It is intended that sources within the spring-build-junit project be
folded into spring-core/src/test/java, pending some Gradle work that
will facilitate sharing test output across subprojects; therefore this
commit should be seen as a temporary solution.

Issue: SPR-9984